You said we cannot know if a guy, divorced remarried, has knowledge of his sin.

Actually YES, WE CAN.
We can judge a tree by the fruits.
With a prudent and balanced judgement, of course. We cannot see directly conscience. But we can see actions, its consequences.

What you are forgetting is that marriage is not an act like others. It’s his own kind. You cannot do marriage by mistake or impulsively. You have to do banns. You have to attend a class. There are steps. You don’ t stumble upon marriage abruptly.

Come on. Don’ t joke.
You cannot say “I did not know”. To make people know is the purpose of the Church “mater et magistra”.

The knowledge that makes a person fully culpable for a sin is not simple knowledge of the fact that the Church teaches something. Sometimes, to fallen sinners, it seems that the Church might be wrong, or that there should be exceptions in certain cases. It is clear that many papal critics take this point of view, for they reject various teachings of Pope Francis, thinking that he has erred. The divorced and remarried are a similar case.

Full knowledge is not the only mitigating factor, Sircigles. A person can have full knowledge and still be in the state of Grace if she is not free enough to act otherwise because of external influences.

“In my understanding, it’s hard to say that a validly married person may lack the full knowledge, and it seems to me that the Pope’s attention is focused on the second subjective condition—deliberate consent. Subjective responsibility for an objectively sinful situation may sometimes be lacking because the freedom of the parties involved is restricted as a result of various factors or circumstances, such as “duress, fear, habit, inordinate attachments,” “affective immaturity, force of acquired habit, conditions of anxiety or other psychological or social factors” (§ 302, quoting the Catechism).

When the Argentinian Bishops, for instance, in their guidelines endorsed by the Pope and declared by him to be of the “authentic Magisterium”, say that “especially when a person believes he/she would incur a subsequent fault by harming the children of the new union, Amoris Laetitia offers the possibility of having access to the sacraments of Reconciliation and Eucharist (cf. footnotes 336 and 351)”, they are clearly referring, I think, to relationships in which one person is either not Christian or not practicing the faith, and also threatening serious consequences, e.g., leaving a civilly remarried spouse and children if they do not consent to sexual relations (which is pretty obvious because a non-Catholic would not understand such a requirement and this would seem to him/her as an absurd intrusion in his/her private life, thus damaging the relationship).

In this case the person might not be guilty of actual mortal sin, for example, if the divorced and remarried Catholic:

1. For serious motives is not able to separate.

2. He/she intends to refrain from acts proper to spouses, even though he/she can’t do that for the reasons explained above.

3. He/she has received the sacrament of Penance with this intention.

If these conditions are met, the divorced and remarried need not need be in the state of actual mortal sin and he or she can be absolved and receive the Holy Eucharist, if he/she receives in such a way as to avoid scandal. And I would like to point out that there is no general invitation made to divorced and remarried to receive, so it remains clear that it is not normal, but an exception for them to receive.”

That is a different kind of question and is not so simple. But I would focus onto full knowlwdge because it is the very crucial point. You see we are arrived to discuss faith.

If a guy knows that Church teachs a thing, and he disagrees with that thing, can you still say that his faith is equal (both subjectively and objectively) to yours? Communion? And what kind of communion is that?

You can claim the case of Orthodoxes. But they have a their own clergy with real Sacraments – which no one else does.